One of the problems in traditional robotics is that the limbs of a robot must be bend resistant so that the position of the end effector can be positioned by summing the vectors from each axis rotation point. To make the limbs bend resistant the most common approach is to manufacture them out of metal. This results in excessively heavy robots that demand a high amount of power during operation. In mobile robotics this renders a relatively low battery time which in turn limits the capabilities of the robot. The weight of the robot limbs can be reduced by introducing more light weighted materials such as carbon fiber, which has a high strength to weight ratio. This on the other hand has the downside that the limbs will be prone to bending. This proposal therefore results in decreased energy demands but also a decrease in precision. Many different methods have been proposed to model robots with bending limbs, also referred to as continuum robots, all resulting in more or less precise 3D positioning results depending on what sensor data is used. Sensors used in continuum robotic limbs make use of force sensors attached to varying parts of the robot (Ref 1). X-Rays and other visual systems have also been used in shape estimation see e.g. (Ref. 2) and (Ref. 3). Positioning using optical fibers have been proposed where three pairs of optical fibers are attached on the robot. Light is emitted through one of the fibers in each pair. The light is directed out from the robots body towards the surrounding walls. The intensity of the light that bounces back from the walls is measured through the other optical fiber in the pair. By a priori knowledge of the surrounding walls the information from the intensity measurements can be used to make a positioning of the robot see (Ref. 4). Such a measurement procedure is however inherently dependent upon the background and will significantly reduce the possible environments where the robot can be successfully used.
The bend of a flexible robotic limb can be determined using bend resistors, where the resistor is fastened on the limbs. Bend resistors exists today in the form of resistive sensors that change the resistance of the device when bent. The resistance is proportional to the bend and can thus be used to estimate the bend of the sensor or the force that is applied to the sensor if other specifications of the sensor are given. One example of this is the Tactilus® Flex by SENSOR PRODUCTS INC. One approach of estimating the bend in a structure element using at least three strain sensors positioned around the structure is disclosed in Ref. 5. The lengthening and shortening of the structure at a specific position where a strain sensor is present can be measured. By using multiple strain sensors the bend of the structure can be estimated. This approach utilizes Bragg-gratings to obtain the estimate.
In Ref 6, there is proposed a sensor for determining the bend of a number of optical fibers bundled together. The optical fibers form part of a colonoscopy camera and are provided with holes on their surface. A measurement of the intensity of light emitted through the optical fibers is compared to an initial intensity level. If the fibers are bent a certain amount of light will escape through the holes and thus leading to a difference in detected intensity contra the emitted intensity. The fact that the optical fibers are bundled together will however negatively affect the precision of the measurements needed to be able to determine the position of a flexible large body subject to substantial forces and torques.
Multi-tubular continuum robotic limbs have been used in robotic applications where weight and material use may be an issue. Generally, the tubes are fastened in solid sectional dividing frames. A multi sectional approach can be used where each sectional divider can be considered a frame. Estimation of the dynamic transformation of each pair of bases is done using modeling in combination with sensor readings and a priori knowledge of external forces. From the dynamic transformation and information about the tubular structure 3D positioning of the end effector and other parts can be made. This proposal however relies upon the a priori knowledge of the applied forces and are therefore somewhat lacking in respect of the precision that is needed to obtain an accurate positioning of robotic limbs.
It is a purpose of the proposed technology to provide alternative methods and apparatuses for the positioning a flexible element such as a robotic limb. This technology aims to at least alleviate the problems within the technical field and at the same time provide accurate positioning of extended flexible elements subject to forces and torques.